﻿using System;
using System.IO;
using System.Text;

/// <summary>
/// FileEncoding 的摘要说明
/// </summary>

/// <summary>
/// 获取文件的编码格式
/// </summary>
public class EncodingType
{
    /// <summary>
    /// 给定文件的路径，读取文件的二进制数据，判断文件的编码类型
    /// </summary>
    /// <param name="FILE_NAME">文件路径</param>
    /// <returns>文件的编码类型</returns>
    public static System.Text.Encoding GetType(string FILE_NAME)
    {
        FileStream fs = new FileStream(FILE_NAME, FileMode.Open, FileAccess.Read);
        Encoding r = GetType(fs);
        fs.Close();
        return r;
    }

    /// <summary>
    /// 通过给定的文件流，判断文件的编码类型
    /// </summary>
    /// <param name="fs">文件流</param>
    /// <returns>文件的编码类型</returns>
    public static System.Text.Encoding GetType(FileStream fs)
    {
        byte[] Unicode = new byte[] { 0xFF, 0xFE, 0x41 };
        byte[] UnicodeBIG = new byte[] { 0xFE, 0xFF, 0x00 };
        byte[] UTF8 = new byte[] { 0xEF, 0xBB, 0xBF }; //带BOM
        Encoding reVal = Encoding.Default;

        BinaryReader r = new BinaryReader(fs, System.Text.Encoding.Default);
        int i;
        int.TryParse(fs.Length.ToString(), out i);
        byte[] ss = r.ReadBytes(i);
        if (IsUTF8Bytes(ss) || (ss[0] == 0xEF && ss[1] == 0xBB && ss[2] == 0xBF))
        {
            reVal = Encoding.UTF8;
        }
        else if (ss[0] == 0xFE && ss[1] == 0xFF && ss[2] == 0x00)
        {
            reVal = Encoding.BigEndianUnicode;
        }
        else if (ss[0] == 0xFF && ss[1] == 0xFE && ss[2] == 0x41)
        {
            reVal = Encoding.Unicode;
        }
        r.Close();
        return reVal;
    }

    /// <summary>
    /// 判断是否是不带 BOM 的 UTF8 格式
    /// </summary>
    /// <param name="data"></param>
    /// <returns></returns>
    private static bool IsUTF8Bytes(byte[] data)
    {
        int charByteCounter = 1;　 //计算当前正分析的字符应还有的字节数
        byte curByte; //当前分析的字节.
        for (int i = 0; i < data.Length; i++)
        {
            curByte = data[i];
            if (charByteCounter == 1)
            {
                if (curByte >= 0x80)
                {
                    //判断当前
                    while (((curByte <<= 1) & 0x80) != 0)
                    {
                        charByteCounter++;
                    }
                    //标记位首位若为非0 则至少以2个1开始 如:110XXXXX...........1111110X　
                    if (charByteCounter == 1 || charByteCounter > 6)
                    {
                        return false;
                    }
                }
            }
            else
            {
                //若是UTF-8 此时第一位必须为1
                if ((curByte & 0xC0) != 0x80)
                {
                    return false;
                }
                charByteCounter--;
            }
        }
        if (charByteCounter > 1)
        {
            throw new Exception("非预期的byte格式");
        }
        return true;
    }
}